There has been extensive use of an image forming apparatus using an electrophotographic process wherein an electrostatic latent image is formed on a photoreceptor (photosensitive drum), and toner is attached thereon, is transferred onto paper and other mediums, and is then fixed. Especially in the image forming apparatus wherein higher image quality and higher resolution are required, as in the field of office printers for high volume printing and on-demand printing apparatuses, a wet type development system has come in use because this system employs a liquid developer that is characterized by reduced toner particle size and minimized toner image disturbances.
One of the proposals made in recent years is an image forming apparatus which uses liquid developer with high viscosity and high concentration which is prepared by dispersing, in high concentration, solid toner made of resin and pigment in an insulative liquid “liquid carrier” such as silicone oil.
When this liquid developer is used for development, generally, a thin layer of the developer on the order of microns is formed on a developer carrier such as a developing roller, and this thin-filmed developer is brought in contact with the photoreceptor as an image carrier, whereby development is performed.
The latent image on the photoreceptor surface is developed with the thin layer of liquid developer and a toner image is formed on the photoreceptor surface. This toner image is transferred onto a recording medium. Alternatively, the toner image is once transferred onto an intermediate transfer member or others and is transferred onto a recording medium. The toner image having been transferred onto the recording medium is pressed and heated by a fixing apparatus and is fixed on the recording medium which is usually a sheet of paper.
The commonly known developing device that develops the latent image on the photoreceptor surface using a liquid developer includes a developer supply section for supplying liquid developer; a thin layer forming roller for forming a thin layer of the supplied liquid and transferring the developer to the developing roller; and a developing roller for carrying the thin layer of liquid developer and developing the latent image on the photoreceptor.
The thin layer forming roller includes one roller or a plurality of rollers such as a supply roller which is dipped in a developer tank as a developer supply section to receive a regulated amount of liquid developer, and a conveyance roller which forms the developer in a thin layer and transfers the developer to the developing roller.
The developing roller and the aforementioned thin layer forming rollers are rotated each in contact with at least one of the other rollers with liquid developer held in between, whereby the liquid developer supplied from the developer supply section is continuously transferred, and a thin layer of the aforementioned developer with a thickness on the order of microns is formed on the developing roller to develop a latent image on the photoreceptor.
For example, the Japanese Patent Application Publication No. 2006-343676 carries the description of the technique on the developing device having the aforementioned structure.
In such a developing device, however, the developer remains on each roller immediately after image formation. If the remaining developer is left for a while, a liquid carrier will evaporate from the developer remaining on the roller, or toner will separate from the liquid carrier, with the result that the percentage of solid toner will increase.
If the remaining developer is left still for a long time as it is, the toner will stick to the roller, and the amount of developer on the roller will not be uniform for the next image forming process. This will cause uneven image density.
In one of the methods proposed to solve this problem, after completion of image formation, for example, the supply roller is separated from the liquid developer in the developer tank not to supply the liquid developer. After that, rollers in contact with one another are driven for a predetermined period of time or longer. In addition, the developer is removed from the roller by a cleaning member (Japanese Patent Application Publication No. 2001-75365, Japanese Patent Application Publication No. 2007-148243 and Japanese Patent Application Publication No. 2008-20761).
This arrangement allows the developer on the supply roller to move to the conveyance roller, and the developer on the conveyance roller to move to the developing roller. Thus, the developer is removed by a cleaning blade of the developing roller. After rotation for a predetermined period of time or longer, there will be no developer on each roller, and there is no concern about the possibility of toner sticking thereto.
Japanese Patent Application Publication No. 2001-75365 discloses a technique of preliminary driving prior to image formation. Japanese Patent Application Publication No. 2007-148243 illustrates the method for removing the developer by driving the rollers, with the developing roller completely separated from the supply roller. Japanese Patent Application Publication No. 2008-20761 describes the method of removing the developer by driving the rollers while applying bias voltage applied between the rollers.
However, according to Japanese Patent Application Publication No. 2001-75365, the proposed method is to perform preliminary driving to level out the accumulated developer on the roller. It is not intended to remove the developer. If the proposed method is aimed at removing the developer, the following problems will arise.
According to the technique disclosed in Japanese Patent Application Publication No. 2007-148243, the proposed method would work well if there were only a developing roller and a supply roller. However, in the case that where are additional rollers provided, unless all those rollers are separated from one another (this arrangement will make removal of the developer difficult), the following problems will also arise.
The method proposed in Japanese Patent Application Publication No. 2008-20761 also has some problems. Regardless of the bias voltages applied between the rollers, since the developer is to be removed by driving the rollers in contact with one other, the following problems will also arise.
To be more specific, to remove the developer, if the supply of developer is suspended and the rollers are driven in contact with one another, the amount of the developer on the peripheral surface of each roller will be reduced as the rollers continue rotating. Then the rollers will rotate in contact with one another, with a reduced amount of developer present on the nip between rollers or without any developer thereon.
A certain contact pressure is applied to the rollers driven in contact with one another in order to restrict the thickness of the thin developer layer during image formation or to transfer the developer from one roller to another. Thus, if rollers are driven in contact with one another having an insufficient amount of developer in the nip between rollers or having no developer at all, the roller surface may be damaged, and more drive torque will be needed.
In particular, the damages on the developing roller surface tend to carry irregular amount of toner or create cleaning problems. This will seriously deteriorate the image quality.
In addition, when the developer on such rollers is removed by cleaning and the rollers start rotating again in the next image formation step, the rollers will be driven in contact with one another having an insufficient amount of developer in the nip between rollers or having no developer at all. This will create the same problem.